This invention relates to a modem and trellis code modulation and more specifically to a 64-State 8D Encoder having a 20-point 2D constellation.
In communication systems, the received signal constellation may be rotated with respect to the transmitted signal constellation, due to phase hits. Phase hits or jumps occurring in the communication medium may result in a rotation of the received signaling alphabets as compared to the initial determination of phase. This ambiguity in phase can cause errors in all subsequently received data and thereby seriously degrade the performance of the system. This rotation may cause the phase of the received signal to jump permanently to a new value. More specifically, if there is a phase hit, the received signal might be rotated by multiples of 90.degree..
It is known in the prior art that encoding the data at the transmitter of a modem using a convolutional encoder and performing maximum likelihood (Viterbi) decoding at the receiver decreases the error rate of the modem at a given signal-to-noise ratio (SNR), assuming that the major impairment is Gaussian noise.
The idea of jointly selecting a trellis code and a signal constellation to optimize performance for band limited channels was first discussed by Ungerboeck in his article entitled "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, Vol. IT-28, January 1982, pp. 55-67. Ungerboeck showed that for 2D codes, almost all of the gain theoretically achievable, by adding redundancy through coding and increasing the number of constellation points, could be gained by using a rate (n-1)/n code and a coded constellation with twice the number of points as the uncoded one. Lee-Fang Wei, in an article entitled "Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space--Part II: Nonlinear Codes", IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 5, September 1984, pp. 672-686, extended Ungerboeck's ideas and created 2D nonlinear trellis codes that are rotational invariant to 90.degree. phase shifts. One of Wei's codes was selected for the V32 CCITT specification. More recently, Wei discovered multi-dimensional trellis codes that give improved performance over the 2D codes. The codes are designed for encoders that accept seven data bits per baud. This work is referenced in another article by Ungerbocck entitled "Trellis-Coded Modulation with Redundant Signal Sets, Part II: State of the Art", IEEE Communications, Vol., 25, No. 2, February 1987, p. 21 and one of the codes is, also, used in the CODEX 19.2 kbps modem.